World's Most worst Nuclear Tragedys

Three Mile Island:

Outside View of the Three Mile Island Nuclear Power Plant Photo Courtesy Nuclear Regulatory Commission

On an island 10 miles from Harrisburg Pennsylvania resides the Three Mile Island Nuclear Power Station. There are two reactors at the plant, dubbed Unit 1 and Unit 2. One of them is inoperable. Unit 2 experienced a partial reactor meltdown on March 28, 1979. A partial nuclear meltdown is when the uranium fuel rods start to liquefy, but they do not fall through the reactor floor and breach the containment systems. The accident which occurred at Unit 2 is considered to be the worst nuclear disaster in US history. Why did it happen? There are many reasons for the accident, but the two main ones are simple human error and the failure of a rather minor valve in the reactor. In the following paragraphs, we will explain how it was possible for the accident to happen and both its psychological and physical effects on the American people.The accident at TMI (Three Mile Island) began at about four in the morning with the failure of one of the valves that controlled coolant flow into the reactor. Because of this, the amount of cool water entering the reactor decreased, and the core temperature rose. When this happened, automatic computerized systems engaged, and the reactor was automatically SCRAMmed. The nuclear chain reaction then stopped. This only slowed the rate at which the core temperature was increasing, however. The temperature was still rising because of residual heat in the reactor and energy released from the decaying fission products in the fuel rods.
Because the pumps removing water from the core were still active, and a valve that controlled the cool water entering the core failed, water was leaving the core, but not coming in. This reduced the amount of coolant in the core. There wasn't enough coolant in the core, so the Emergency Core Cooling System automatically turned on. This should have provided enough extra coolant to make up for the stuck valve, except that the reactor operator, thinking that enough coolant was already in the core, shut it off too early.
There still wasn't enough coolant, so the core's temperature kept increasing. A valve at the top of the core automatically opened to vent some of the steam in the core. This should have helped matters by removing the hot steam, but the valve didn't close properly. Because it didn't close, steam continued to vent from the reactor, further reducing the coolant level. The reactor operators should have known the valve didn't close, but the indicator in the control room was covered by a maintenance tag attached to a nearby switch. Because the operators didn't know that the valve had failed to close, they assumed that the situation was under control, as the core temperature had stopped rising with the first venting of steam from the core. They also thought that the coolant had been replaced in the core, because they didn't know that the pump outlets were closed. A few minutes later the core temperature began to rise again, and the Emergency Core Cooling System automatically switched on. Once again, an operator de-activated it, thinking the situation was under control. In reality, it was not.
Soon, because of the coolant lost through the open valve at the top of the reactor, the core temperature began to rise again. At this point the fuel rods started to collapse from the intense heat inside the core. The operators knew something was wrong, but didn't understand what it was. This was about 5 minutes after the initial valve failure. It took almost 2 hours for someone to figure out that the valve releasing steam at the top of reactor hadn't closed properly. During those 2 hours, precious coolant continued to be released from the reactor a meltdown was underway. At approximately 6AM, an operator discovered the valve at the top of the core was open and closed it.
During the day hydrogen gas began to accumulate inside the reactor and caused an explosion later in the afternoon. This explosion did not damage the containment systems, however. Two days later, the core was still not under operator control. A group of nuclear experts were asked to help evaluate the situation. They figured out that a lot of hydrogen gas had accumulated at the top of the core. This gas could have exploded, like the explosion on the first day of the accident, or it could have displaced the remaining coolant in the reactor, causing a complete nuclear reactor meltdown. No one really knew what to do about the hydrogen build-up. A hydrogen recombiner was used to remove some of the hydrogen, but it was not very effective. However, hydrogen also dissolves in water, which is what the coolant was composed of. Thus, over time the hydrogen that had collected at the top of the core completely dissolved in the coolant. Two weeks later the reactor was brought to a cold shutdown and the accident was over.No one was directly injured as a result of the accident. However, some radioactive gas and water were vented to the environment around the reactor. At one point, radioactive water was released into the Susquehanna river, which is a source of drinking water for nearby communities. No one is really sure what effects these radioactive releases might have had on people living near the power plant.

Chernobyl:

About 80 miles (130 km) north of Kiev, in what is now the Ukraine, is located the Chernobyl nuclear power plant. At this plant the worst reactor disaster to ever occur took place on April 26, 1986. It happened largely because normal reactor operations were suspended; an experiment was to take place in the reactor. As a result, normal safety guidelines were disregarded, and the accident occurred. However, as with most accidents of this type, it was a result of many small mistakes adding up to create a catastrophe. In the following paragraphs, we will outline just how the event transpired:Early in the day, before the test, the power output of the reactor was dropped in preparation for the upcoming test. Unexpectedly, the reactor's power output dropped way too much, almost to zero. Because of this drop, some control rods were removed to bring the power back up. (As you recall from the fission power text, the more control rods there are in a reactor, the more free neutrons are absorbed and the less fissioning that goes on. So, more control rods means less energy and power output.) The reactor's power output raised up, and all appeared to be normal.
More preparation for the test began later when two pumps were switched on in the cooling system. They increased water flow out of the reactor, and thus removed heat more quickly. They also caused the water level to lower in a component of the reactor called the steam separator. Because of the low level of water in the steam separator, the operator increased the amount of feed water coming into it, in the hopes that the water level would rise. Also, more control rods were taken out of the reactor to raise internal reactor temperature and pressure, also in the hopes that it would cause the water level in the steam separator to rise. The water level in the steam separator began to rise, so the operator adjusted again the flow of feed water by lowering it. This decreased the amount of heat being removed from the reactor core.
Because many control rods had been removed and the amount of heat being taken from the core by the coolant had been reduced, it began to get very hot. Also, there was relatively low pressure in the core because the amount of incoming water had been decreased. Because of the heat and the low pressure, coolant inside the core began to boil to form steam.
The actual test began with the closing of the turbine feed valves. This should have caused an increase in pressure in the cooling system, which in turn would have caused a decrease in steam in the core. This should have lowered the reactivity in the core. Thus, the normal next step when closing the turbine feed valves was to retract more control rods, increasing reactivity in the core. This is what the operator at Chernobyl did. The only problem was that in this case there was no increase in pressure in the cooling system because of the earlier feed water reduction. This meant that there was already a normal amount of steam in the core, even with the turbine feed valves closed. Thus, by retracting more control rods to make up for a reduction in steam that didn't happen, the operator caused too much steam to be produced in the core.
With the surplus of steam, the reactor's power output increased. Soon, even more steam was being produced. The operator realized there was a problem and SCRAMmed the reactor, completely disabling all fission reactions. However, it was too late. The temperature and pressure inside the reactor had already risen dramatically, and the fuel rods had begun to shatter.
After the fuel rods shattered, two explosions occurred as a result of liquid uranium reacting with steam and from fuel vapor expansion (caused by the intense heat). The reactor containment was broken, and the top of the reactor lifted off. With the containment broken, outside air began to enter the reactor. In this particular Soviet reactor, graphite was used as a moderator instead of water. (water was the coolant) As air entered the core, it reacted with the graphite. Graphite is essentially just carbon, so oxygen from the air chemically combined with the carbon to form CO (carbon monoxide). Carbon monoxide is flammable and soon caught fire. The fire emitted extremely radioactive smoke into the area surrounding the reactor. Additionally, the explosion ejected a portion of the reactor fuel into the surrounding atmosphere and countryside. This fuel contained both fission products and transuranic wastes.
During the days following the accident, hundreds of people worked to quell the reactor fire and the escape of radioactive materials. Liquid nitrogen was pumped into the reactor core to cool it down. Helicopters dumped neutron-absorbing materials into the exposed core to prevent it from going critical. Sand and other fire-fighting materials were also dropped into the core to help stop the graphite fire. All in all, over 5000(metric) tons of material were dropped into the core. After the fires were brought under control, construction of what is called "the sarcophagus" began. The word "sarcophagus" is usually used to describe the elaborate coffins the ancient Egyptians used to entomb their dead. In this case, the sarcophagus is a structure erected from about 300,000 metric tons of concrete that surrounds the reactor. It was designed to contain the radioactive waste inside. It has served its purpose well, but, now, ten years after the accident, several flaws have been found in it. Holes have begun to appear in the roof, allowing rainwater to accumulate inside. This water can corrode the structure, further weakening it. Also, birds and other animals have been seen making homes in the sarcophagus. If they should ingest radioactive material, they could spread it around the countryside. Additionally, with time the sarcophagus has become worn down. It is conceivable that an intense event like an earthquake, tornado, or plane crash directly on the sarcophagus could lead to its collapse. This would be catastrophic, as radioactive dust would once again rain down on the surrounding areas. Scientists and engineers are working on ways to repair or replace the structure.
One of the great tragedies of the accident was that the Soviet government tried to cover it up. Clouds of fallout were traveling towards major population centers such as Minsk, and no one was warned. No one outside the Soviet Union knew about the accident until two days later, when scientists in Sweden detected massive amount of radiation being blown from the east.
The effects of the disaster at Chernobyl were very widespread. The World Health Organization (WHO) found that the radiation release from the Chernobyl accident was 200 times that of the Hiroshima and Nagasaki nuclear bombs combined. The fallout was also far-reaching. For a time, radiation levels in a Scotland were 10,000 times the norm. 30 lives were directly lost during the accident or within a few months after it. Many of these lives were those of the workers trying to put out the graphite fire and were lost from radiation poisoning. The radiation released has also had long-term effects on the cancer incidence rate of the surrounding population. According to the Ukrainian Radiological Institute over 2500 deaths resulted from the Chernobyl incident. The WHO has found a significant increase in cancer in the surrounding area. For example, in 1986 (the year of the accident), 2 cases of childhood thyroid cancer occurred in the Gomel administrative district of the Ukraine (this is the region around the plant). In 1993 there were 42 cases, which is 21 times the rate in 1986. The rate of thyroid cancer is particularly high after the Chernobyl accident because much of the radiation was emitted in the form iodine-131, which collects in the thyroid gland, especially in young children. Other cancer incidence rates didn't seem to be affected. For example, leukemia was no more prevalent after the accident than before.What caused the accident? This is a very hard question to answer. The obvious one is operator error. The operator was not very familiar with the reactor and hadn't been trained enough. Additionally, when the accident occurred, normal safety rules were not being followed because they were running a test. For example, regulations required that at least 15 control rods always remain in the reactor. When the explosion occurred, less than 10 were present. This happened because many of the rods were removed to raise power output. This was one of the direct causes of the accident. Also, the reactor itself was not designed well and was prone to abrupt and massive power surges.

1. Fukushima Daiichi Nuclear Power Plant Status

Tables 1 - 4 track progress for Units 1 - 4 towards fulfilling the three basic safety functions of the IAEA safety standards: prevention of criticality, removal of decay heat and mitigation of radioactive releases. The tables replace the three-colour table that was used previously. The charts are cross-referenced to the Tokyo Electric Power Company (TEPCO) "Roadmap" plan to bring the nuclear reactors and the spent fuel pools at the Fukushima Daiichi plant to a stable cooling condition and to mitigate radioactive releases.

On 17 May 2011, TEPCO provided a status report against the TEPCO "Roadmap" showing progress since the Roadmap was issued on 17 April 2011. While the basic policy and targets defined in the Roadmap remain, several changes were made to account for new information obtained and progress made to date.

On 13 May TEPCO commenced the preparatory work for the installation of a cover for the reactor building of Unit 1. The reactor building cover will be installed as an emergency measure to prevent the dispersion of radioactive substances until mid- to long term measures, including radiation shielding, are implemented.

TEPCO has reported that information obtained after calibration of the reactor water level gauges of Unit 1 shows that the actual water level in the Unit 1 reactor pressure vessel was lower than was indicated, showing that the fuel was completely uncovered. The results of provisional analysis show that fuel pellets melted and fell to the bottom of reactor pressure vessel at a relatively early stage in the accident.

TEPCO reported that "most part of the fuel is considered to be submerged in the bottom of reactor pressure vessel and some part exposed." TEPCO also reported that leakage of cooling water from the reactor pressure vessel is likely to have occurred. However, TEPCO considers that the actual damage to the reactor pressure vessel is limited, on the basis of the temperatures now being measured around the reactor pressure vessel.

The results of the analysis are provisional; TEPCO will continue to conduct investigations. Similar analyses will be conducted for Units 2 and 3 when radiation levels allow calibration of the instrumentation.

Nitrogen gas is still being injected into the containment vessel in Unit 1 to reduce the possibility of hydrogen combustion inside the containment vessel.

In Units 1, 2 and 3 fresh water is being continuously injected both via the feed water system lines and the fire extinguishers lines into the reactor pressure vessel; temperatures and pressures remain stable.

To protect against potential damage as a result of future earthquakes, TEPCO started work on 9 May to install a supporting structure for the floor of the spent fuel pool of Unit 4. TEPCO has formulated the hypothesis that the damage to the Unit 4 building could have been caused by hydrogen generated at Unit 3 that flowed into Unit 4.

Fresh water is being injected as necessary into the spent fuel pools of Units 1 - 4. Water supply from concrete pump trucks is being gradually replaced by the Fuel Pool Cooling and Clean-up system in Units 1 to 3. However, closed loop cooling has not been yet established.

Stagnant water with high levels of radioactivity in the basement of the turbine buildings of Units 1 and 3 is being transferred to the condensers, the radioactive waste treatment facility, the high-temperature incinerator building and temporary storage tanks. Stagnant water in the basement of the turbine building of Unit 6 is being transferred to a temporary tank. Countermeasures against the outflow of water to the sea and to prevent and minimize the dispersion of radionuclides in water have been put in place.

Full-scale spraying of anti-scattering agent is continuing at the site with the use of both conventional and remote controlled equipment.

2. Radiation Monitoring

The daily monitoring of the deposition of caesium and iodine radionuclides for 47 prefectures is continuing. Since 17 May, deposition of I-131 has not been observed. Low levels of Cs-137 deposition were reported in a few prefectures on a few days since 18 May; the reported values range of from 2.2 to 91 Bq/ m2 for Cs-137.

Gamma dose rates values for all 47 prefectures are reported daily by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. On 31 May the gamma dose rate reported for Fukushima prefecture was 1.5 µSv/h. In all other prefectures, reported gamma dose rates were below 0.1 µSv/h; with a general decreasing trend. Meanwhile, the decrease of the gamma dose rate has slowed down, since the short-lived radionuclides have decayed away.

Gamma dose rates reported specifically for the monitoring points in the eastern part of Fukushima prefecture, for distances of more than 30 km from the Fukushima Daiichi plant, showed a general decreasing trend, ranging from 0.1 µSv/h to 17 µSv/h, as reported for 31 May.

On-site measurements at the west gate of the Fukushima Daiichi plant indicate the presence of I-131 and Cs-137 in the air in the close vicinity of the plant (within approximately 1 km). The concentrations in air reported for 29 May were about 3 Bq/m3 for I-131 and about 9 Bq/m3 for Cs-137. The values observed in the previous days show daily fluctuations with an overall decreasing tendency.

Protective Actions

In April, the Government of Japan announced protective actions to reduce the external exposure to the population beyond a distance of 30 km from the Fukushima-Daiichi Nuclear Plant. NISA has reported that the evacuation of the "Planned Evacuation Zones" within Iitate village and Kawamata town commenced on 15 May. Confirmation of completion of the evacuation is awaited.

Food Monitoring and Food Restrictions

Food Monitoring (Reported from 19 to 31 May)

Food monitoring data were reported from 19 to 31 May by the Ministry of Health, Labour and Welfare for a total of 818 samples collected in 18 different prefectures. Most of the monitoring continues to be concentrated in Fukushima prefecture, where 328 out of the 818 samples (over 40%) were collected.

Analytical results for 766 samples (over 93%) of the 818 samples indicated that Cs-134 and Cs-137 or I-131 were either not detected or were below the regulation values set by the Japanese authorities. However, 52 samples were above the regulation values for radioactive caesium and/or iodine.

In Fukushima prefecture, five samples of fishery products collected on 16 and 17 May; one sample of unprocessed tea leaves collected on 17 May; three samples of shiitake mushrooms and nine samples of bamboo shoots collected on 19 May; five samples of seafood collected on 20, 21 and 23 May, and; one sample of Japanese apricot, two samples of shiitake mushrooms and seven samples of bamboo shoots collected on 26 May were above the regulation values for Cs-134/Cs-137. One sample of algae collected on 21 May was also above the regulation values for Cs-134/Cs-137 and I-131.

In Chiba, Gunma, Ibaraki and Tochigi prefectures, eighteen samples of unprocessed raw tea leaves collected on 17, 19, 24 and 26 May were above the regulation values for Cs-134/Cs-137.

Food Restrictions

Consolidated and updated information on food restrictions in Fukushima prefecture were reported on 30 May by the Ministry of Health Labour and Welfare indicating that restrictions on the distribution of bamboo shoots were lifted in the Hirata-Mura area. However, restrictions remain in effect on the distribution of raw unprocessed milk, turnips, bamboo shoots and ostrich fern in specific areas of the prefecture. Restrictions on the distribution and consumption of sand lance fish (the whole prefecture) and specific non-head type (e.g. spinach) and head-type leafy vegetables (e.g. cabbage), flower head brassicas (e.g. broccoli, cauliflower) and shiitake mushrooms (specific areas of the prefecture) also remain in effect.

In Ibaraki prefecture there is a continuing restriction on the distribution of spinach produced in the cities of Kitaibaraki and Takahagi.

3. Marine Monitoring

The marine monitoring programme is carried out both near the discharge areas of the Fukushima Daiichi plant by TEPCO at 22 locations and at off-shore stations by MEXT on 16 stations. The radioactive contamination of the marine environment had occurred by aerial deposition and by continuing discharges and outflow of water with various level of radioactivity from the four damaged reactors at Fukushima Daiichi.

Seawater Monitoring

The activity concentrations of I-131, Cs-134 and Cs-137 in seawater close to the Fukushima Daiichi plant at the screen of Unit 2 have been measured every day since 2 April. Concentrations of Cs-134 and Cs-137 decreased from of more than 100 MBq/L initially to less than 5 kBq/L on 7 May but increased again to levels of around 20 kBq/L at the 16 May and to about 10 kBq/L on the 17 May. Since then the concentrations dropped slowly to less than 2 kBq/L but increased to about 5 kBq/L on 29 May. The levels of I-131 are varying significantly and the activity ratio to radio-caesium is not constant. On 28 and 29 May the concentrations were around 20 kBq/L. The variability of I-131 relatively to the radio-caesium concentrations could be an indication of retention of caesium by the zeolite sandbags in place, which would have almost no effect on iodine or further production of decay products in the reactor.

Monitoring of the marine environment is performed by TEPCO on the near field area and by MEXT at off-shore sampling positions. The monitoring of MEXT includes also measurement of ambient dose rate in air above the sea, analysis of ambient dust above the sea, analysis of surface samples of sea water and analysis of samples of sea water collected at 10 m above the sea bottom and in a mid-layer as well at a few locations for sediments. On most of the offshore stations I-131, Cs-134 and Cs-137 reached levels below the applied detection limit of 10 Bq/L. There will be a further decrease of the concentration during the propagation of contaminated waters in the sea. The activity found in surface sediments at the near shore stations close to the reactors was between 24 and 320 Bq/kg for Cs-137 in the middle of May. The activity in sediments decreases with distance, but is also highly dependent upon the sediment type. The contamination of marine sediments indicates the enrichment of radio-caesium on particulate matter and its removal from the water column into the sea floor.

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